1. Field of the Invention
The invention relates to a ballast for driving multiple lamps in a liquid crystal display (LCD). More specifically, the invention relates to a method for sensing a short circuit condition in the ballast transformers outputs to the multiple lamps.
2. Description of the Related Art
In some lamp ballasts, it is desirable to sense a short circuit condition on the output and take an action. This action could be to reduce the power level or shut down entirely in order to avoid circuit breakdown or other possible catastrophic situations. The conventional method to achieve this is to sense the current in the output (hence the lamp current) and compare to a threshold. When the reference level is exceeded, the action can be taken. This current sensing can be implemented in several ways, for instance by inserting a resistor in series with the load and sensing the voltage drop across it.
A difficulty with the state of the art is that it requires a measurement of current. It may be undesirable because the sensing resistor dissipates too much power. It may also be difficult if the circuit inherently limits the current to the load. An example would be an LCD backlight inverter based on the Alpha IC (UBA2010). The secondary winding of the transformers has enough leakage inductance that its impedance limits the current if the load is shorted. The circuit is self protecting, but one may still wish to sense the shorted condition and shut down for safety reasons.
What is needed is a ballast where a short circuit condition is sensed by a method other than measuring lamp (load) current or output voltage directly.
The invention is a lamp ballast for energizing a lamp which has a new method for sensing a short circuit condition. The lamp ballast has a transformer that provides an output voltage to the lamp. The transformer has a primary winding, a secondary winding, and a third sense winding. The third winding has an output which provides an indication of transformer output voltage. A reference voltage circuit creates a reference voltage providing an indication of a non-short circuited lamp ballast condition. A comparator compares the reference voltage and the output of the third sense winding. The comparator provides an indication if the third sense winding output voltage is below the reference voltage thus indicating a short in the output of the lamp ballast.
The method of winding the transformer improves the third sense winding output voltage signal. The transformer is wound on a bobbin with multiple sections. The transformer secondary is wound on multiple sections of the bobbin with one section being a low voltage section. The sense winding is wound under the secondary in the bobbin secondary low voltage section.
Several circuits are necessary to implement the above circuit for indicating a short in the output of the lamp ballast. First, the ballast has a dimming circuit during which time the lamp voltage is reduced substantially. The ballast has a track and hold circuit which cooperates with the voltage of the sense winding at the comparator to hold the voltage above the reference voltage so as to prevent an indication of a short in the lamp voltage.
Secondly, during startup the transformer output is zero and would have an indication of a short circuit. A special circuit is provided to delay any indication of a short circuit until the lamp ballast circuit is powered up. During startup the sense winding voltage at the comparator gradually increases during a preheat mode. A preheat logic signal applies a low voltage to the reference voltage circuit during the preheat mode. This prevents the reference voltage from exceeding the sense winding voltage thus preventing a short circuit indication during the preheat mode.